In conventional techniques, a cycloolefin polymer is formed by the addition polymerization of cycloolefin monomers. Because the cycloolefin polymer has better transmittance, it is mainly used as a raw material of optical elements, such as lens or light-guiding plates. However, the cycloolefin polymer does not have a benzene ring in its structure. Hence, compared with the aromatic compounds, the cycloolefin polymer has very poor flame-resistance.